In 2012, 14.1 million people were diagnosed with cancer worldwide and 8.2 million died of cancer. In the United States, around 40% of all people will be diagnosed with cancer during their lifetime. Despite receiving the best treatment available, 44% of those Americans will die from cancer.
Cancer is the result of a cell dividing without limitation. Healthy cells have checkpoints that prevent unlimited cell division. A few examples of these checkpoints are nutrient availability, DNA damage and contact inhibition (i.e. a cell comes into contact with another cell). Additionally, most cells can replicate only a finite number of times and thus are programmed to die after a particular number of cell divisions.
Cancer is the result of a cell overcoming these built-in checkpoints and proliferating beyond control. This uncontrolled proliferation leads to the formation of a tumor. There are two types of tumors, benign and malignant. Benign tumors are incapable of crossing natural boundaries between tissue types. Malignant tumors, on the other hand, are capable of invading nearby tissue or entering the bloodstream and metastasizing to a different location. Only malignant tumors are considered cancerous. It is this ability to infiltrate and metastasize that makes cancer such a deadly disease.
To further complicate the fight against cancer, malignant tumors have distinct cell types. One particularly troublesome type is cancer stem cells (“CSC's”). CSC's are capable of self-renewing and differentiating into the distinct types of cancer cells found in a malignant tumor. Thus, CSC's are a primary factor in the metastatic ability of a tumor. CSC's often survive radiation and chemotherapy. It is hypothesized that recurrence of cancer after radiation and chemotherapy is the result of the inability of radiation and chemotherapy to kill all CSC's combined with the ability of CSC's to establish a new tumor.
A particularly troublesome type of cancer is brain cancer. Brain cancers, such as high-grade gliomas, are often treated with surgery followed by radiation therapy. Surgery for brain tumors is often very complicated. The surgeon must remove the tumor without damaging any nearby brain tissue that could result in physical or cognitive disabilities. Often the surgeon is incapable of removing the boundaries of the tumor that contact the healthy tissue. Radiation therapy is often used to kill these remaining cancer cells. However, radiation doses are limited by the potential damage to healthy brain tissue. Unfortunately, brain cancer is usually chemotherapy resistant. This resistance is largely attributable to the blood-brain barrier (“BBB”). The BBB is a physical barrier that separates the fluid surrounding the brain from blood cells and other components in the blood stream. Most anti-cancer drugs are unable to cross the BBB.
One method of treating brain cancer is to inhibit the growth of new blood vessels that are necessary for tumor size progression. Bevacizumab marketed under the trademark Avastin® (Avastin is a registered trademark of Genentech, Inc.) is used to stop and even reverse tumor vascularization. However, Rich J., and colleagues, Canc Res, 2006, 66, 7843, found that when Avastin® was used to treat a glioma stem cell derived brain tumor it resulted in hypoxia and a lowered pH. Sathornsumetee S., Phase II trial of bevacizumab and erlotinib in patients with recurrent malignant glioma, Neuro-Oncol, 2010, Dec. 12(12), 1300-1310. Hypoxia and low pH are both known to cause CSC propagation and can promote CSC-driven tumor recurrence.
Chemotherapy is a term used to describe a particular type of cancer treatment that includes using cytotoxic anti-cancer drugs. Cytotoxic drugs used during chemotherapy can be broken down into several main categories including alkylating agents, antimetabolites, anti-tumor antibiotics, topoisomerase inhibitors, and mitotic inhibitors. Cytotoxic anti-cancer drugs typically cause cell division to cease and thus affect healthy tissue as well as cancerous tissue. Alkylating agents stop cancer cell division by damaging the DNA of the cancer cell. Some common alkylating agents used to treat cancer are nitrogen mustards (e.g. cyclophosphamide (Cytoxan®; Cytoxan is a registered trademark of Baxter International), nitrosoureas, alkyl sulfonates, triazeines, and ethylenimines. Platinum drugs, such as cisplatin and carboplatin, work similarly to alkylating agents. Antimetabolites stop cancer cell division by inhibiting DNA and RNA synthesis. Some common antimetabolites used to treat cancer are 6-mercaptopurine, gemcitabine (Gemzar®; Gemzar is a registered trademark of Eli Lilly and Company), methotrexate and pemetrexed (Alimta®; Alimta is a registered trademark of Eli Lilly and Company). Topoisomerase inhibitors stop cancer cell division by inhibiting topoisomerase enzymes from separating the DNA for replication. Some common topoisomerase inhibitors are topotecan, irinotecan, etoposide, and teniposide. Mitotic inhibitors stop cancer cell division by inhibiting key cell division enzymes. Some common mitotic inhibitors are taxanes (e.g. paclitaxel (Taxol®; Taxol is a registered trademark of Bristol-Myers Squibb Company) and docetaxel (Taxotere®; Taxotere is a registered trademark of Aventis Pharma SA)), epothilones, and vinca alkaloids.
One disadvantage of all of these anti-cancer drugs is the damage that they do to healthy tissue. Because the drugs treat cancer by inhibiting normal cell function, healthy tissue that also relies on constant cell division such as blood cells, mucosal surfaces and skin, can be severely damaged as well. This damage results in significant morbidity and can limit the amount of chemotherapy that can safely be delivered. Examples of side effects that occur during chemotherapy treatment include low blood count, hair loss, muscle, and joint pain, nausea, vomiting, diarrhea, mouth sores, fever, and chills. To overcome this problem drugs are being developed that affect proteins and cellular functions that occur only in cancer cells. Some of these specific cancer drugs are imatinib (Gleevec®; Gleevec is a registered trademark of Novartis AG), gefitinib (Iressa®, Iressa is a registered trademark of AstraZeneca UK Limited), sunitinib (Sutent®; Sutent is a registered trademark of C.P. Pharmaceuticals, International C.V.), and bortezomib (Velcade®; Velcade is a registered trademark of Millennium Pharmaceuticals, Inc.). However, these drugs are not approved for the treatment of all cancer types and are universally associated with the development of treatment resistance. Thus, a need exists in the art for an anti-cancer drug delivery vehicle that can deliver potent, effective, broad spectrum anti-cancer drugs to cancer cells including CSC's while avoiding substantial uptake of the drug by healthy cells. Additionally, the anti-cancer drug delivery vehicle should be able to cross the BBB and deliver the anti-cancer drug to cancer cells of the brain.
Currently, there are few chemical compounds that preferentially target cancer cells. One such compound is CLR1404. Generally, CLR1404 is a promising new tumor-selective diagnostic imaging agent used to monitor the treatment response of several tumor treatment modalities. Radioiodinated CLR1404, a second-generation phospholipid ether (“PLE”) analog with the following structure,
has displayed remarkable tumor selectivity in 55/60 xenograft, orthotopic and transgenic cancer and cancer stem cell derived animal models making the core molecule an ideal platform for an anti-cancer drug delivery vehicle. See U.S. Pat. No. 8,535,641; U.S. Patent Application Publication No. 2014/0030187 and Weichert, J. P., et al., Alkylphosphocholine analogs for broad-spectrum cancer imaging and therapy, Sci Transl Med, 2014, Jun. 11, 6(240), 240ra75; each of which are incorporated by reference herein in its entirety.
What is not known is whether a compound that is selectively sequestered and retained by cancer cells and cancer stem cells is capable of delivering an anti-cancer drug to these same cells. Further, it is not known whether this compound is also capable of transporting anti-cancer drugs across the BBB to treat brain cancers. Finally, it is unknown whether this or similar compounds can cause the cancer cell to retain the anti-cancer drug in sufficient quantities and for a sufficient period of time to eradicate the tumor and prevent further growth and metastasis. The present invention adapts the CLR1404 core molecule for use as an anti-cancer drug delivery vehicle capable of targeting the anti-cancer drug to cancer cells and cancer stem cells including brain cancer cells. Further, the compounds of the present invention are retained in cancer cells.